Developing JAXB Applications Using EclipseLink MOXy, Release 2.4
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Mapping Composite Key Relationships

If the objects that you want to map have multi-part keys (that is, a combination of fields that determines uniqueness), you can use EclipseLink's @XmlKey and @XmlJoinNodes to set up this relationship.

One or more @XmlKey annotations can be used to declare the primary keys in a given class. For a single key, either @XmlID or @XmlKey can be used. For composite primary keys, multiple @XmlKey annotations can be used, or a single @XmlID can be combined with one or more @XmlKey annotations.


Composite Keys can be useful when using JAXB to map JPA entities. For more information see Converting JPA entities to/from XML (via JAXB).

In Example 7-8, each Employee has one manager but multiple reports, and Employees are uniquely identified by the combination of their id and name fields.

Example 7-8 Using the @XmlKey and @XmlJoinNodes Annotations

package example;
import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.*;
public class Employee {
    private Integer id;
    private String name;
    @XmlJoinNodes( {
        @XmlJoinNode(xmlPath = "manager/@id", referencedXmlPath = "@id"),
        @XmlJoinNode(xmlPath = "manager/@name", referencedXmlPath = "@name") })
    public Employee manager;
    @XmlJoinNodes( {
        @XmlJoinNode(xmlPath = "report/@id", referencedXmlPath = "@id"),
        @XmlJoinNode(xmlPath = "report/@name", referencedXmlPath = "@name") })
    public List<Employee> reports = new ArrayList<Employee>();

Example 7-9 shows how to define this mapping information in EclipseLink's OXM metadata format.

Example 7-9 Sample XML Mapping

<java-type name="Employee">
        <xml-attribute java-attribute="id" xml-id="true" />
        <xml-attribute java-attribute="name" xml-key="true" />
        <xml-join-nodes java-attribute="manager">
            <xml-join-node xml-path="manager/@id" referenced-xml-path="@id" />
            <xml-join-node xml-path="manager/@name" referenced-xml-path="@name" />
        <xml-join-nodes java-attribute="reports" container-type="java.util.ArrayList">
            <xml-join-node xml-path="report/@id" referenced-xml-path="@id" />
            <xml-join-node xml-path="report/@name" referenced-xml-path="@name" />

This would produce the following XML:

   <employee id="1" name="Jane Doe">
      <report id="2" name="John Smith"/>
      <report id="3" name="Anne Jones"/>
   <employee id="2" name="John Smith">
      <manager id="1" name="Jane Doe"/>
   <employee id="3" name="Anne Jones">
      <manager id="1" name="Jane Doe"/>